The increased interest in environmental awareness, and a reluctance to rely on non-renewable sources of energy, has led to greater usage of renewable energy systems, e.g. solar power, wind power, thermal power, and tidal power. The latter involves the installation of turbine generators in an area of tidal flow, and converts the energy of the tides into electrical energy.
With reference to FIGS. 1 and 2, an example hydroelectric turbine generator is described in PCT Application No. PCT/EP2007/006258. The generator 10 is of a direct-drive shaftless permanent magnet generator construction. The generator 10 comprises a ring-shaped stator 12 and a rotor 14, the stator 12 having a plurality of coils 17 (FIG. 2) located about the circumference of the stator 12. The rotor 14 comprises an inner rim 16, which defines an open centre. The rotor 14 further comprises an array of generally radially extending blades 18 captured between the inner rim 16 and an outer rim 20. The rotor 14 further comprises an array of magnets 21 (FIG. 2) disposed about the outer rim 20. The stator 12 concentrically surrounds the rotor 14, with the plurality of coils providing a magnetic flux return path for the magnets.
The generator 10 is positioned in an area of tidal flow, such that the motion of the seawater through the generator 10 acts on the blades 18, resulting in rotation of the rotor 14 within the stator 12. The motion of the magnets relative to the coils of the stator 12 causes the magnetic field generated by the magnets to induce an EMF in each of the coils. These induced EMFs provide the electrical power output from the turbine generator 10.
As there is increased utilisation of such tidal power generators, it becomes of greater importance to be able to accurately and effectively control the operation of the generators, in particular ensuring that the turbine generator is operating at its optimum “tip-speed ratio”. The “tip-speed ratio” is the ratio between the rotational speed of the tip of a blade of the turbine and the actual velocity of the tidal flow, and is generally turbine-design dependent, with each turbine having an optimum tip-speed ratio for the optimum power generation by that turbine. Thus, the speed of operation of the turbine should be varied to ensure optimum output power for the level or tidal power available. Also, sometimes it can be necessary to “run back” the generated power output, dependent on the requirements of the main power grid.
In standard electrical generator systems, the amount of power extracted from generators can be controlled by varying the voltage or frequency of operation. This control is generally executed local to the generator. The electrical generator and associated controller are usually easily accessible for commissioning, servicing tests, maintenance and settings adjustments. In addition to a controller, renewable energy generators often locate power conversion and monitoring equipment in the vicinity of the generator, in order to immediately convert output power to a fixed voltage and frequency, and in order to monitor the condition of each individual generator.
The transmission systems that transport output power operate over long distances at a high voltage, using a voltage and frequency that is fixed to a certain value within a relatively small tolerance gap. Accordingly, the voltage output by a generator is normally stepped up to transmission levels close to the generators using a transformer.
Harnessing tidal energy presents significant challenges with respect to maintenance of the turbine in order to ensure continuing and efficient operation in the harsh submarine environment. The installation and decommissioning of submarine turbine generators is relatively expensive, and it is not economic to carry out regular maintenance on the various components in the system. Also, it is not feasible to include sensitive electronic equipment, power conversion and/or cooling systems local to the submarine generator (i.e. components that require regular servicing and routine maintenance in order to maintain long term reliability).
Therefore, it is an object of the invention to provide an alternative system and method for controlling the rotational speed of a hydroelectric turbine, and consequently the output of a hydroelectric turbine generator, which eliminates the need for controlling and monitoring components located local to the submarine generator.